Two-speed hydraulic servomotor system



Nov. 17, 1959 F. H. KINDL TWO-SPEED. HYDRAULIC SERVOMOTOR SYSTEM lieFiled Feb. 21, 1957 H/S A TTORNE)" TWO-SPEED HYDRAULIC SERVOMOTOR SYSTEMFred HflKindl, Ballston Spa, N.Y., assignor to General Electric Company,a corporation of Nevv'York Application February 21, 1957, Serial No.41,626 4cm. Cl. 121-41 This invention relates to a hydraulic servomotormechanism, particularly to such a mechanism which is used forpositioning the stem of a steam valve in 'a prime mover such as anelastic fluid turbine.

' The servo mechanism described particularly herein is illustratedascontrolling the'operation of an intercept valve which is used tostopthe flow of motive fluid from the reheater section of a steam generatorto the succeeding stages of a reheat type turbine.

In the operation of a steam turbine it is desired to have quick openingand closing times of the intercept valve. The conventional way ofcontrolling the intercept valves is by a separate oil pressure systemwhich actuates a single acting servomotor to move the valves to the openor closed position. The servomotor is normally springbiased in onedirection and actuated in the other direction by oil under pressure, thesupply of which is regulated by a pilot valve.

The intercept valve is normally of the balanced type =which balances outthe effect of the steam flowing through the valve to prevent the steamfrom having any effect on the opening and closing movement of the valve.With this arrangement, the valve is under the control of the hydraulicservomotor system employed in such cases. While this arrangement hasbeen generally satisfactory, experience has shown that the effectivepressure over the downstream face of the valve disk can be much higherthan pressures further downstream and also higher than the balancechamber pressure which in most instances tends to be only slightly abovedownstream pressure.'

,trol disk 1d controlling the flow between inlet 1h and outlet-1i has apiston portion Ie, the upper surface of 2,912,962 Patented Nov. 17,

Generally stated, the invention is practiced byproviding a hydraulicservo mechanism for controlling a steam turbine valve, which servomechanism includes a regulator for controlling the movement of the servomechanism; and means interposed between the regulator and the servomechanism to limit the operating speed of the servo'mechanism in onedirection while at the same timemaintaining a rapid movement of theservo mechanism in the opposite direction.

Referring now to the drawing, the intercept valve 1 is illustrateddiagrammatically as comprising a housing 1a supporting the bushingassembly 1b, in which is slidably disposed the valve stem 10. Because itmust perform accurate throttling functions, the valve 1 illustrated isof a pressure-balanced type. In such a valve the flow conwhich isexposed to the downstream pressure communicated to chamber 1 by apressure balancing conduit 1g. Thus, the pressure on the bottom surfaceof disk 1d jis'etfectively balanced by the pressure on piston 1eand'reduces to a minimum the force required to move the valve to thefully open position. The operation of the intercept valve 1 iscontrolled by a servomotor 2 which is connected through rod 2a to stem10 of intercept valve 1.

Rod 2a defines at its upper portion a valve stem sealing shoulderidentified 2g which engages with a cooperating series of openings 3a,3b, 3c, 3d, 32, and 3} adapted to When this responsive to turbine speed(not shown).

-into chambers 2d, 2e.

seat in bushing 1b to limit the upward movement of valve stem 1c. Theservomotor is controlled by a pilot valve 3 which is under the controlof some automatic device The pilot valve is-so arranged that upon apredetermined overspeed condition it is operated in a manner to bedescribed hereinafter to close the' intercept valve. A control mechanismof this type is described more particularly in the U'.S. Patent2,747,373, issued May 29, 1956, on an "application of M. A. Eggenbergeret al., and assigned to the same assignee as the present invention.

Referring now to the servomotor 2 in detail, it comprises a cylindricalhousing 2b containing piston 20 which is secured to piston rod 2a andwhich divides housing -2b Piston 2c is biased in a downward direction bycompression spring 2 located in chamber 2d. The flow of fluid toservomotor 2 is controlled by pilot valve 3 which is a conventionalpiston valve having a receive inlet conduit 5, outlet conduits 6, 7 anddrain conduits 9a, 9b respectively. Within the pilot valve 3' is adouble piston arrangement 3g, 3h which are axially dis- .posed on rod31'.

Rod 3i is connected to lever 15a which movement is controlled by link15b which in turn is con- Accordingly, it is an object of this inventionto provide an improved hydraulic servomotor system for actuating avalve, which incorporates special flow controlling means to limit theopening movement of the valve to prevent the imposition of excessivestresses on the valve stem when the valve reaches its maximum openposition Y while permitting rapid movement of the valve in the closingdirection.

A further object is to provide a novel servo mechanism specially adaptedfor use witha balanced valve which provides for rapid closing movementof the valve but which prevents rapid opening movement and thus tendsto. prevent excessive loading of the valve stem which smay result inexcessive stretching or breakage of the valve stem.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawing, whichdiagrammatically illustrates a hydraulic servomotor incorporating theinvention and arranged to operate the intercept valve of a-stcamturbinee nected up to a device (not shown) responsive to turbinespeed. The pistons 3g, 3h are located to control the flow of oil atports 3b, 3c, respectively. Conduit 6 interconnects ports 3b and chamber2e in servomotor 2, and conduit 7 connects up port 30 and chamber 2d.Thus, when rod 3i is moved downwardly, inlet conduit 5 which containsorifice 5a to limit the flow therethrough, is connected up with conduit7 and conduit 6 is connected up with drain conduit9a to drain chamber 2eto a sump (not shown). When the pilot valve is in this position, the oilfrom inlet 5 acts on the piston 20 to move the intercept valve 1 in theclosing direction. Downward movement into chamber 2d past the rod 2a.

In accordance with the invention, the drain conduit 9 isformedintheshape of 'an inverted'U so'thatapressure head 71 is set up toprovide a back pressure in conduits 9a, 9b. Interconnecting chamber 2dand conduit 9a through pilot valve is conduit 8. Located in conduit 8 ischeck valve 12 which permits flow in the direction leading :from conduit9 to chamber Zdbut prevents flow from taking place in the oppositedirection. Thus, when the pressure in chamber 2d drops below that inconduit 9 oil will flow through check valve 12, conduit 8, and intochamber 2d. Conduit 7 which interfollows.

Upon receipt of a signal from the pre-emergency speed --relay (notshown) calling for movement of the valve 1 in a closing direction, linkb, and leveriSa are moved downwardly to move the valve control rod 3i tointerconnect inlet conduit 5 with conduit 7, and conduit 6 with drainconduit 9a to admit high pressure oil to chamber 2d and drain fluid fromchamber 2e respectively. It ,the signal requires only a small motion,the valve stem 1c and piston rod 2a moves in the closing direction onlyuntil the return linkage 15c, 15d, 15c restores the pilot valve to itson-port position, at which time the valve closing movement of rod 2astops. If, however, the turbine loses full load, the floating lever 15aand pilot valve rod 31' drops far enough so that the pilot valve 3 willstay off port until valve 1 is closed. When this occurs, the fluid fromchamber 2e is rapidly drained through conduits 6, and 9a; and thenspring 2 combined with the fluid pressure in chamber 2d forces valve 1closed very rapidly. It can be appreciated that this will require oil inchamber 211 at a rate much greater than can be supplied through conduit7 and flow limiting orifice 13, with the result that a partial vacuumcould exist in chamber 2d. However, at this time check valve 12 opens topermit oil to flow directly from the chamber 2e through conduit 6, pilotvalve 3, conduit 8 and check valve 12 to chamber 2d. This flow takesplace due to the back pressure existing in conduit 9 resulting from theconfiguration thereof. The provision of this supplemen tary fluidprevents a partial vacuum from occurring which, if permitted wouldresult in actually lengthening the closing time of the intercept valvewhich would be undesirable. It can be appreciated by'one skilled in theart of steam turbines that it is important that the control valves 'beclosed as quick as possible on overspeed to prevent damage to theturbine.

It remains to note that the oil seal 11 prevents air from leaking intochamber Zn! in the event a partial vacuum exists in chamber 2d. Thus,

since the'oil in the servomotor system is recirculated a large flow ofhigh pressure oil from the fluid pressure source (not shown) is notrequired to obtain the desired open and closing times of the valve'l.

When it is desired to open the intercept valve 1, the

"pilot valve rod 3i is moved upwardly through actuation tion, the upwardmovement of intercept valve 1 is retarded by the orifice 13 which limitsthe rate at which fluid can flow from chamber 2d to drain conduit 9b.Thus,

pressure-i8 built up in chamberld to produce a downward force sufficientto overcome any steam forces which may exist tending to blow the valveopen. It should be noted that no fluid can flow through check valve 12at this time. The reduction in the upward speed of the valve 1 preventsthe valve stem 10 from being stretched or broken when the sealingshoulder 2g engages bushing 11). l

Thus it can be seen that with the aid of this invention, the interceptvalve 1 will be closed by the servo mechanism in the desired minimumtime, but will be prevented from rapidly jerking open by the temporarycushioning effect of the fluid contained in the servo mechanism, whichcushioning effect is the result of restricting the flow of fluid outfrom the chamber 2d when the valve 1 is moved in the opening direction.

While a single embodiment of the invention has been described in detailherein, it will be obvious to those skilled in the art that many changesand substitutions of equivalents might be made. For example, it may bedesirable to connect conduit 8 to a separate source of fluid underpressure and not rely on the configuration of drain conduit 9 to providea back pressure in conduit 8 so that'fluid is available in conduit 8when thevalve 1 is moved'to a closed position. In addition, a reliefvalve could be located in conduit 9 to maintain a back pressure therein.Also, while the invention is illustrated as being applied to a balancedvalve,.it will be obvious to those skilled in the art thattheinventionis not so limited.

It is, of course, desired to cover by the appended claims all suchmodifications as fall within the true spirit and scope of the invention.A i

What I claim asnew and desire to secure by Letters Patent of'the UnitedStates is:

l. in a hydraulic servo mechanism for positioning an output member byactuating means including a servomotor comprising a housing, an outputpiston connected to said output member and dividing the housing intofirst and second chambers, a source of operating fluid under pressure, apilot valve for controlling the flow of fluid from said source to theservomotor, and conduit means interconnecting the source, pilotvalve,-and first and second chambers whereby upon movement of the pilotvalve to direct the flow of fluid from the source to the first chamberthe output piston is movedin a first direction tending to reduce thesize of the second chamber, and upon movement of the pilot valveto-direct the flow of fluid to the second chamber the piston is moved inthe opposite direction tending to reduce the size of the first chamber,means biasing said piston in the first direction, and means dependent onthe setting 'of the pilot valve for moving the output member at a firstspeed in the first directionand at a second slower speed in the oppositedirection, said means including a first-conduit connecting the pilotvalve and first chamber, means in the first conduit for restricting theflow therethrough,a second conduit connecting said second chamber andsaid first chamber, first valve means in said second conduit forpermitting flow in the direction from the second chamber to said firstchamber, and second valve means between said first valve means and thesecond chamber arranged to be positively controlled by said pilot valvemovement to admit fluid to the first valve means.

2. Apparatus of the character described which includes a source forsupplying fluid under pressure to a servo mechanism, the servo mechanismcomprising a reduce the size of'the second chamber, a pilot valvefor-regulating the flow of fluid between said source-and said first andsecond chambers, first conduit means interconuecting said source and thepilot-valve, second conduit means interconnecting the pilot valve andthe first chamber, third conduit means interconnecting the pilot valveand the second chamber, fourth conduit means interconnecting SaKlSECOlldchamber and the first chamber, first valve means in said fourth conduitfor permitting flow in the direction between said second chamber andfirst chamber, flow limiting means in said second conduit for retardingthe fiow of fluid between the pilot and the first chamber, and secondvalve means arranged to be positively controlled by the pilot valvemovement to admit fluid from the second chamber to the fourth conduit,whereby upon movement of the pilot valve to direct the flow of fluidfrom the source to the first chamber and drain the fluid from the secondchamber, the piston is moved in the first direction at a faster ratethan when the piston is moved in the opposite direction when the pilotvalve is moved to a position to supply fluid to the second chamber anddrain fluid from the first chamber.

3. A hydraulic system for actuating a control valve for supplying motivefluid to a turbine, the combination of a hydraulic servomotor comprisinga housing containing a piston dividing the housing into first and secondchambers and having an output member secured thereto, spring means insaid first chamber for biasing the piston in a first direction, a sourceof fluid under pressure for supplying fluid to said servomotor, a pilotvalve for controlling the flow of fluid between said source of pressureand servomotor, the pilot valve including a housing defining an axialbore and a pressure fluid inlet port, drain ports spaced axially oneither side of the inlet port, outlet ports defined between the inletport and each of said drain ports, and a reciprocating flow controlmember disposed in said bore and comprising an axially slidable memberwith at least two axially spaced lands disposed in said bore incooperation with said outlet ports, a first conduit means connectingsaid source with said inlet port, second and third conduit meansconnecting said outlet ports with the first and second chambers of saidhousing respectively, flow limiting means in said second conduit forrestricting the flow of fluid therethrough, fourth conduit meansconnecting said second chamber and said first chamber, check valve meansin said fourth conduit means for limiting'flow through said fourthconduit means in the direction from said second chamber to said firstchamber, whereby upon movement of the flow control member to a positionwhich interconnects the first conduit means with the second conduitmeans, and the third conduit means to drain, the piston will. be movedin a first direction at a first speed in response to the spring means insaid first chamber and the fluid supplied from said inlet, and saidsecond chamber through said second and fourth conduit meansrespectively, and upon movement of the flow control member to a positionwhich interconnects the first conduit means with the third conduit meansand the second conduit means to drain, the piston will be moved in theopposite direction at a second speed slower than said first speed due tothe means in said second conduit which restricts the flow of fluid fromsaid first chamber.

4. In a hydraulic servo mechanism for positioning an output member byactuating means including a servomotor comprising a housing, an outputpiston connected to the output member and dividing the housing intofirst and second chambers, resilient means in said first chamber forbiasing the piston in a first direction, a source of fluid underpressure for supplying fluid to said servomotor, a pilot valve forcontrolling the flow of fluid between said source of pressure andservomotor, the pilot valve including a housing defining an axial boreand a pressure fluid inlet port, drain ports spaced axially on eitherside of the inlet port, outlet ports defined between the inlet port andeach of said drain ports, and a reciprocating flow control memberdisposed in said bore and comprising an axially slidable member with atleast two axially spaced lands disposed in said bore in cooperation withsaid outlet ports, a first conduit means connecting said source withsaid inlet port, sec ond and third conduit means connecting said outletports with the first and second chambers of said housing respectively,flow limiting means in said second conduit for restricting the flowtherethrough, fourth conduit means for draining the fluid from saiddrain ports, means for providing a positive pressure in said fourthconduit means, fifth conduit means connecting said fourth conduit meanswith said first chamber, check valve means in said fifth conduit meansfor permitting flow in the direction between said fourth conduit meansand said first chamber, whereby upon movement of said pilot valve in adirection to interconnect the source of fluid under pressure and saidfirst chamber the piston is moved in the first direction at a firstspeed and upon movement of the pilot valve to interconnect the source offluid under pressure and said second chamber the piston is moved in theopposite direction at a second speed substantially less than said firstspeed.

References Cited in the file of this patent UNITED STATES PATENTS563,304 Ongley July 7, 1896 1,912,184 Ferris May 30, 1933 2,191,792 HillFeb. 27, 1940 2,622,866 Johnsen Dec. 23, 1952 2,628,731 Reuter Feb. 17,1953 2,646,025 Deardorff July 21, 1953

